Fix a heap overflow in loadpgrd.cpp

==781106==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x6040000055c0 at pc 0x0000008245a2 bp 0x7fffddc0fbb0 sp 0x7fffddc0fba8
READ of size 4 at 0x6040000055c0 thread T0
    #0 0x8245a1 in ESM::Pathgrid::load(ESM::ESMReader&, bool&) /home/jvoisin/dev/openmw/openmw/components/esm/loadpgrd.cpp:106:44
    #1 0x51126a in load(Arguments&) /home/jvoisin/dev/openmw/openmw/apps/esmtool/esmtool.cpp:389:21
    #2 0x50aa40 in main /home/jvoisin/dev/openmw/openmw/apps/esmtool/esmtool.cpp:216:20
    #3 0x7f436666ecb1 in __libc_start_main csu/../csu/libc-start.c:314:16
    #4 0x44834d in _start (/home/jvoisin/dev/openmw/openmw/build_fuzz_asan_master/esmtool+0x44834d)

0x6040000055c0 is located 0 bytes to the right of 48-byte region [0x604000005590,0x6040000055c0)
allocated by thread T0 here:
    #0 0x4f205d in operator new(unsigned long) (/home/jvoisin/dev/openmw/openmw/build_fuzz_asan_master/esmtool+0x4f205d)
    #1 0x821971 in __gnu_cxx::new_allocator<int>::allocate(unsigned long, void const*) /usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/ext/new_allocator.h:115:27
    #2 0x821971 in std::allocator_traits<std::allocator<int> >::allocate(std::allocator<int>&, unsigned long) /usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/alloc_traits.h:460:20
    #3 0x821971 in std::_Vector_base<int, std::allocator<int> >::_M_allocate(unsigned long) /usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/stl_vector.h:346:20
    #4 0x821971 in std::vector<int, std::allocator<int> >::reserve(unsigned long) /usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/vector.tcc:78:22
    #5 0x821971 in ESM::Pathgrid::load(ESM::ESMReader&, bool&) /home/jvoisin/dev/openmw/openmw/components/esm/loadpgrd.cpp:89:40
    #6 0x51126a in load(Arguments&) /home/jvoisin/dev/openmw/openmw/apps/esmtool/esmtool.cpp:389:21
    #7 0x50aa40 in main /home/jvoisin/dev/openmw/openmw/apps/esmtool/esmtool.cpp:216:20
    #8 0x7f436666ecb1 in __libc_start_main csu/../csu/libc-start.c:314:16

SUMMARY: AddressSanitizer: heap-buffer-overflow /home/jvoisin/dev/openmw/openmw/components/esm/loadpgrd.cpp:106:44 in ESM::Pathgrid::load(ESM::ESMReader&, bool&)
Shadow bytes around the buggy address:
  0x0c087fff8a60: fa fa fd fd fd fd fd fa fa fa fd fd fd fd fd fa
  0x0c087fff8a70: fa fa 00 00 00 00 00 fa fa fa fd fd fd fd fd fa
  0x0c087fff8a80: fa fa fd fd fd fd fd fa fa fa fd fd fd fd fd fa
  0x0c087fff8a90: fa fa fd fd fd fd fd fa fa fa 00 00 00 00 00 fa
  0x0c087fff8aa0: fa fa 00 00 00 00 00 fa fa fa 00 00 00 00 00 fa
=>0x0c087fff8ab0: fa fa 00 00 00 00 00 00[fa]fa fa fa fa fa fa fa
  0x0c087fff8ac0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c087fff8ad0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c087fff8ae0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c087fff8af0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c087fff8b00: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
  Shadow gap:              cc
==781106==ABORTING